Composite antenna feed

ABSTRACT

A composite antenna feed subsystem concentrated in a small area at the prime focus of the parabola of a satellite parabolic reflector accomodates a plurality of frequency bands. The arrays comprising the subsystem are mounted on the top cover of a communication module. A multimode horn is arranged at the center of the subsystem axis which functions at X- and C-band frequencies, and a cross-array consisting of individual elements form the S-band feed, with one arm of the S-band array containing an element mutually shared with the L-band array. Provision is also made for UHF frequencies, and a dipole arrangement for VHF frequencies is arranged around the S-band arms.

United States Patent [1 1 Fletcher et al.

[ COMPOSITE ANTENNA FEED -[76] Inventors: James C. Fletcher,Administrator of [22] Filed: Sept. 21,1971 [2]] Appl. No.: 182,399

[451 July 17, 1973 3,474,452 l0/l969 Bogner ..343/726 PrimaryExaminer-Rudolph V. Rolinec Assistant ExaminerSaxfield Chatmon, Jr.Attorney-R. F. Kempf. John R. Manning cl all.

[57] ABSTRACT A composite antenna feed subsystem concentrated in a smallarea at the prime focus of the parabola of a satellite parabolicreflector accomodates a plurality of frel5 2] Cl 343/725 2 4 2 5 quencybands. The arrays comprising the subsystem are 51 I t Cl 6 2/1/00mounted on the top cover of a communication module. i 727 A multimodehorn is arranged at the center of the subs g"; 893 system axis whichfunctions at X- and C-band frequencies, and a cross-array consisting ofindividual elements 5 6 f Ci form the S-band feed, with one arm of theS-band array 1 Re erences ted containing an element mutually shared withthe L-band UNITED STATES PATENTS array. Provision is also made for UHFfrequencies, and 3,482,248 12/1969 Jones, Jr. 343/727 a dipolearrangement for VHF frequencies is arranged 2,953,782 9/1960 Byatt343/726 X around the S-band arms. 3,623.l 11 11/1971 Provencher....343/797 3,611,389 10/1971 .Coors 343/730 13 Claims, 3 Drawing Figures I5(ZigLFEED 5 2 arm) 2 I6 I2 I 9 l t f I] I I I iiiiss f l Ill r i F/ /j n(4PL\IES) I 7 I I l k a A[ 3 J S i r I 7 I JL I I H I/ s i l I i/PATENTEBJUU Hm 3. 747. 1 1 1 SEE 1 0F 2 INVENTOR VITO J. JAKSIYSATTORNEY PAIENIEUJUU H875 3,747, 1 1 1 sum 2 u; 2

GROUND PLANE 3,747,l ll

COMPOSITE ANTENNA FEED ORIGIN OF THE INVENTION The invention describedherein was made in the performance of work under a NASA contract and issubject to the provisions of Section 305 of the National Aeronautics andSpace Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

BACKGROUND OF INVENTION 1. Field of Invention The invention relates to aprime focus antenna feed subsystem which serves as the interface betweenthe multimode, multi-frequency subsystem of a transponder and aparabolic reflector of the antenna. It has particular utility in thefield of antenna feeds at microwave communication wavelengths and forbroadband RF field measurement systems, especially in conjunction withthe NASA Applications Technology Satellite (ATS).

2. Prior Art Multi-frequency feed subsystems having simultaneoustransmit and receive functions are known in the prior art but thoseinvolved relate to multiple antennas of the same type such as multiplehorns. Prior art feed subsystems also require rotating joints andmechanical devices for beam pointing.

SUMMARY OF THE INVENTION The apparatus according to this inventionprovides several feeds designed to perform a diversity of functions.Thus, applicant provides an antenna feed subsystem capable of handlingX-band, C-band, S-band, L- band, UHF and VHF frequencies.

For example, the S-band feed provides incremental beam scanning in twoorthogonal planes and also simultaneous lobing (monopulse) operation.The L-band feed generates a fan-shaped beam by illuminating the entirereflector with uniform phase in one plane and only a portion of thereflector in the orthogonal plane. The feed elements are arranged insuch a manner that minimum interaction results between the variousfeeds.

The feed is concentrated in a small area at the prime focus of theparabolaof the parabolic reflector, and exhibits a high degree ofradiation efficiency and minimal interaction between elements. Thedisclosed embodiment shows the composite feed system comprising anintegral part of the top cover of the communications subsystem of theATS F&G satellite, providing simplicity of assembly and test and theshortest possible RF cable lengths.

- The composite feed subsystem comprises a multimode horn at the centerof the subsystem axis which functions at X- and C-band frequencies. Aplurality of arms consisting of individual elements form the S-bandarray and are centered about the multimode horn, with one arm of theS-band array containing an element mutually shared with the L-bandarray. Provision is also made for UHF and VHF frequencies, the VHFportion comprising a particular array also considered to be part of theinvention.

The disclosed composite feed subsystem provides certain improvementsover prior art multi-frequency arrays, such as:

l. Repeatability of electrical performance of like elements 2. Low costfabrication 3. Minimum weight 4. High reliability necessary ofspacecraft hardware S. Broad frequency coverage 6. Medium densitypackaging 7. Use of simple radiating elements The invention provides abroad frequency spectrum (UHF to X-band) of transmit/receive capability,while eliminating the rotating joints and mechanical devices commonlyassociated with prior art feed systems for beam pointing.

A BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view of thecommunications module showing the composite feed system according to theinvention, mounted on the cover thereof;

FIG. 2 is an isometric view showing the VHF feed e1 ement in greaterdetail;

FIG. 3 is an isometric view showing a typical arrangement of thecrossed-dipole elements comprising the S- and L-band and UHF arrays ofthe composite feed system.

DETAILED DESCRIPTION OF THE DISCLOSURE FIG. 1 is an isometric view of anEarth Viewing Module (EVM) showing the communications subsystemlocation. The composite feed system according to the invention ismounted on the ground plane of the communications module and comprises aprime focus antenna feed subsystem which serves as the interface betweenthe multimode, multi-frequency transponder subsystem and the parabolicreflector in the ATS F&G satellite. The subsystem is capable of handlingX-band, C-band, S-band, L-band, UHF and VHF frequencies concentrated ina small area at the prime focus of the parabola, with a high degree ofradiation efficiency and mini mal interaction between elements. Thefrequency ranges associated with the bands are:

Bands Frequency Range X 7.25 8.4 GHz C 5.9 6.4 GHz S 1.8 2.3 GHz L 1.51.65 GHz UHF 840 860 MHz VHF 136 MHz The X-band feed should generate anextremely narrow high-gain beam on the order of 0.3 to provide accurateantenna pointing and evaluation of reflector performance. The, antennapointing may be achieved in a linearly polarized monopulse systemincorporated in the Xband feed.

The X-band feed comprises a center-mounted multimode horn 1 having thinmetal walls arranged in a square horn configuration. The radiatingaperture is of reduced cross-section to allow for closer spacing of theelements comprising the S-band array described hereafter. The reducedsection is dielectric-loaded to support the necessary modes and to shapethe primary radiation pattern.

The manner of feed excitation is not shown herein because it does notrelate to the composite feed subsystem comprising the invention. Thisstatement also applies to the excitation of other band and frequencyfeeds described hereafter. For illustrative purposes, however, the hornmay be excited from a conventional four-port comparator consisting ofwaveguide hybrids through a four-port rectangular-to-square transitionto generate the modes needed for monopulse operation 3,747,]lll

within the horn structure, the combination of the modes producing-thesum and difference primary patterns.

The C-band feed makes use of a unique design wherein the X-band,muIti-mode horn is shared at C- band frequencies. This providescoincidence between the C-band beam and the X-band sum channel beam. TheC-band feed uses the high-gain of the reflector to receive low-levelsignals radiating from the earth in the 5,900-6,40O MHz frequency band.Two orthogonal, linear and circular polarizations are incorporated toallow analysis of potential Radio Frequency Interference (RFI) problems.At C-band frequencies, the feed may be excited through two shunt ports(not shown) located on opposite walls of the horn, and the horn is thusexcited with two orthogonal modes.

Multimode horn 1, shared between the X- and C- bands, is shown in FIG. 1as being centered at the center of the composite feed system. The S-bandfeed portion of the composite feed system comprises an array consistingof 32 cavity-backed crossed-dipole tumstile elements mounted in across-array coincident with the composite feed axis. With respect toFIG. 1, the S-band array comprises arms 2, 3, 4 and arranged inorthogonal relationship in the same plane about the composite feed axis.The innermost elements of each arm may form a monopulse feed which canbe used for single satellite tracking, although this is not a specificrequirement of the composite feed system. The illustrated cross-arrayarrangement of the 32 tumstile elements comprising the S-band arrayprovides the advantages of simplicity and cost reduction.

The S-band feed enables generation of beams to provide communicationsbetween low-altitude satellites including Apollo and Nimbus. Because ofvariations in the orbits of these satellites, the feed must be designedto generate incrementally scanned beams to permit simultaneouscommunications to the two satellites. In the S-band array, each of the32 turnstile elements is connected to a diplexer and separate transmitand receive switching networks which enable a remote selection of feedelements to provide an independent feedphase center for generating 32secondary beams for the communications coverage in two orthogonal planesfrom the composite feed system axis, in an approximate range of :L-7.5.

In order to locate all feed elements in the focal plane,

the S-band array is elevated above the ground plane as shown in FIG. 1.The S-band diplexers and switching matrices (not shown) are locatedinside the communications module to provide a location having goodthermal control for solid state components.

The third element 6 of arm 3 of the Sband array is shared with elementscomprising the L-band array. FIG. 1 shows the L-band array as comprisinga total of seven cavity-backed, crossed-dipole elements mounted in aline-array 7 to form the fan beam, and a separate cavity-backed,crossed-dipole element 8 to form the pencil beam. Line array 7 isparallel to anns 2 and 4 of the S-band array, and perpendicular to arms3 and 5 thereof. The individual L-band elements are similar toindividual S-band elements except that they are larger in size as shownin FIG. 1. In order to provide identity of elements comprising theL-band array, the third innermost element 6 of the S-band array is ofthe same size as the remaining elements comprising line array 7 andelement 8 producing the pencil beam.

The L-band feed array illuminates the reflector with an ellipticalprimary pattern to produce a normal amplitude and constant phasecharacteristics in one plane, while in the orthogonal plane, only aportion of the reflector is illuminated with a constant phase energy.-Asymmetric reflector illumination generates a fanshaped secondary beamhaving an approximate halfpower beamwidth defined by the wedge of by1.1. Because of the displacement of the feed from the composite feedaxis, the secondary beam is squinted approximately 3 off the antennaaxis, and beam steering is accomplished by tilting the spacecraft. Thepencil beam, produced by element 8, is symmetrical and as discussedabove, is generated by using a separate single-cavity backed,crossed-dipole element to minimize switching losses.

The L-band feed illuminates the reflector in a manner which produces anelliptical beam designed to cover (one-half) of the hemisphere fromsynchronous altitude, i.e., the North Atlantic. This beam is requiredfor the PLACE Experiment which will provide communications to aircrafttraversing this high-density route. Because of the uncertainty inperformance of the aircraft equipment, the high-gain pencil beam isprovided as a backup.

The UHF feed comprises four cavity-backed crosseddipole radiators 9, 10,11 and 12 which form an array clustered around the center of thecomposite feed axis. The crossed-dipole radiators 9 through 12 areinterposed between arms 2 through 5 of the S-band array. The elementsare reduced in size to approximately 0.4 wavelength in order to allowfor closer spacing between the phase centers, thus obtaining morefavorable reflector illumination.

UHF feed generates a circularly polarized on-axis beam, which iscoincident with the X-band monopulse axis. The on-axis beam eliminatesasymmetry, which is always a problem in feed design. The UHF feedilluminates the reflector to provide a symmetrical beam on the order ofapproximately 3. This finds particular utility in the experimentdesigned to transmit educational television signals to remote areas ofAsia, including the country of India.

A VHF feed also comprises part of the composite feed system to providecapability of monopulse acquisition of the ground station and ahigh-gain link for certain telemetry and command data. As shown in FIG.1, it comprises eight quarter-wavelength (It/4) loops or stubs 13through 20 arranged to comprise four dipoles mounted near the outermostends of adjacent arms of the S-band array. Thus, loops l9 and 20comprise a dipole element arranged near the outermost end of arm 4, forexample.

FIG. 2 shows in greater detail, the elements comprising the VHF feed.Each radiating element (20, for example) extends perpendicularly upwardfrom the ground plane in a plane parallel to the wall of the adjacentS-band array for length 21, which is substantially equal to the heightof the top edge of the wall above the ground plane, as shown in FIGS. 1and 2. The radiating element 20 then angles out and up from the wall forlength 22, and then extends out from the wall in a perpendiculardirection thereto, and in a plane parallel to the ground plane forlength 23. Length 21 and a por tion of length 22 of the radiatingelement, are surrounded by a coaxial cable to to prevent interferencewith the adjacent S-band array arm. Dielectric support 24 functions tosupport the outer portion of length 23 from the ground plane. Radiatingelement 20, in association with radiating element 19, thus forms adipole element. The remaining pairs of radiating elements arestructurally and functionally similar to radiating elements 19 and 20and, for this reason, are not described herein in detail.

A typical arrangement of the crossed-dipole elements comprising the S-and L-band and UHF arrays is shown in FIG. 3 of the drawings. The S- andL-band arrays may comprise an integral unit having walls and separatingpartitions defining plurality of cavities housing the crossed-dipoleelements. Lip 35 extends within and around the cavity to supportfiberglass board 36 which is securely attached to the lips. Theradiating arms 37 are mounted in orthogonal relationship to each otherto form the crossed-dipole elements. It is intended that FIG. 3 serveonly as one illustrative embodiment of a conventional crossed-dipoledesign and it is to be understood that other equivalent structures maybe substituted therefor.

The composite feed has a fixed relationship with the prime(longitudinal) axis (axis perpendicular to the EVM cover and alsocoincident with the antenna system axis). Adjustment capability isprovided at the junction of the EVM support truss and parabolicreflector hub to properly relate the feeds and the reflector. Further,to properly locate the feed phase center at the focal plane of thereflector, axial adjustment is provided at the junction of each trussleg and the reflector hub. Adjustment is also provided to accuratelyalign the focal plane and focal axis of the reflector with respect tothe feeds. The adjustment elements are not shown because they areconventional in the art and are discussed herein merely to indicate thatsuch adjustments may be provided.

The composite feed system provides for the elimination of all rotatingjoints and mechanical devices in the feed system for beam pointing.Specific applications of a composite antenna may not require one or moreof the bands or frequencies disclosed herein, and the correspondingelements may therefore be eliminated, thus allowing further optimizationof the remaining feeds without departing from the scope of theinvention.

I claim: l. A composite antenna feed subsystem operative at a pluralityof frequency bands comprising:

a first feed shared by first and second frequency bands,

a second feed for a third frequency band having an array of four armsarranged in orthogonal tumstile relationship centered around the centerof the first feed,

a third feed for a fourth frequency band having a plurality of radiatorelements arranged in a square form array centered around the center ofthe first feed, individual elements of the third feed being interposedbetween adjacent arms of the second feed,

a fourth feed for a fifth frequency band having a plurality of elementsarranged in a line array and including an element mutually shared withan arm of the second feed, and

a fifth feed for a sixth frequency band having an array of elementsarranged near the ends of the arms of the second feed located outermostfrom the center of the first feed, the first, second, third, fourth andfifth feeds being mounted to a common ground plane structure. I 1

2. The composite antenna feed subsystem recited in claim 1 wherein thefirst feed comprises a multimode horn centered at the prime axis of thesubsystem.

3. The composite antenna feed subsystem recited in claim 2 wherein thesecond feed comprises a plurality of crossed-dipole elements in each armof the array.

4. The composite antenna feed subsystem recited in claim 3 wherein thefourth feed comprises a plurality of crossed-dipole elements arranged ina line array parallel to two opposite arms of the second feed.

5. The composite antenna feed subsystem recited in claim 4 wherein thefourth feed further comprises a single crossed-dipole element mountedadjoining the line array.

6. The composite antenna feed subsystem recited in claim 5 wherein thefifth feed array of elements comprises dipoles arranged near theoutermost end of each of the arms of the second feed.

7. The composite antenna feed subsystem recited in claim 11 for use inan EVM satellite as an interface between the transponder and parabolicreflector, wherein the common ground plane structure is the top cover ofthe EVM satellite.

8. A composite feed subsystem mounted on the top cover of an EVM of asatellite for use as an interface between a transponder and parabolicreflectorcomprising:

a plurality of feed arrays for different frequency bands mounted aboutthe prime axis of the EVM top cover,

a VHF feed array having a dipole element mounted at substantially thecenter and near the edge of each side of the EVM top cover;

wherein one of the plurality of feed arrays comprises four arms arrangedin orthogonal tumstile relationship about the prime axis, each armhaving an outermost end located substantially at the center and near theedge of a different side of a substantially rectangular EVM top cover,the VHF feed having first and second coacting radiating elements mountednear said outermost ends of each of the four arms to form four dipoles.

9. The composite antenna feed subsystem recited in claim 9 wherein eachof the arms of said one of the plurality of feed arrays comprises aplurality of crosseddipole elements.

10. The composite antenna feed subsystem recited in claim 9 wherein atleast a portion of the first and second radiating elements locatednearest the arms of said one of the plurality of feed arrays issurrounded by a coaxial cable.

11. In an antenna system, a first VHF integral radiating clementcomprising:

a first length extending in a perpendicular direction relative to aground plane of reference,

a second length extending from the end of the first length in adirection which forms an obtuse angle with the first length,

a third length extending from the end of the second length in adirection substantially perpendicular to the first length,

a dielectric support mounted between a portion of the third lengthnearest its free end and the ground plane,

the first, second and third lengths and the dielectric support defininga plane substantially perpendicular to the ground plane.

12. The antenna system as recited in claim 11 wherein the ground planeof reference comprises the top cover of an EVM satellite, and furthercomprising:

a second VHF integral radiating element spaced from and arranged inmirror image relationship to the first radiating element, operative tofunction there-

1. A composite antenna feed subsystem operative at a plurality offrequency bands comprising: a first feed shared by first and secondfrequency bands, a second feed for a third frequency band having anarray of four arms arranged in orthogonal turnstile relationshipcentered around the center of the first feed, a third feed for a fourthfrequency band having a plurality of radiator elements arranged in asquare form array centered around the center of the first feed,individual elements of the third feed being interposed between adjacentarms of the second feed, a fourth feed for a fifth frequency band havinga plurality of elements arranged in a line array and including anelement mutually shared with an arm of the second feed, and a fifth feedfor a sixth frequency band having an array of elements arranged near theends of the arms of the second feed located outermost from the center ofthe first feed, the first, second, third, fourth and fifth feeds beingmounted to a common ground plane structure.
 2. The composite antennafeed subsystem recited in claim 1 wherein the first feed comprises amultimode horn centered at the prime axis of the subsystem.
 3. Thecomposite antenna feed subsystem recited in claim 2 wherein the secondfeed comprises a plurality of crossed-dipole elements in each arm of thearray.
 4. The composite antenna feed subsystem recited in claim 3wherein the fourth feed comprises a plurality of crossed-dipole elementsarranged in a line array parallel to two opposite arms of the secondfeed.
 5. The composite antenna feed subsystem recited in claim 4 whereinthe fourth feed further comprises a single crossed-dipole elementmounted adjoining the line array.
 6. The composite antenna feedsubsystem recited in claim 5 wherein the fifth feed array of elementscomprises dipoles arranged near the outermost end of each of the arms ofthe second feed.
 7. The composite antenna feed subsystem recited inclaim 1 for use in an EVM satellite as an interface between thetransponder and parabolic reflector, wherein the common ground planestructure is the top cover of the EVM satellite.
 8. A composite feedsubsystem mounted on the top cover of an EVM of a satellite for use asan interface between a transponder and parabolic reflector comprising: aplurality of feed arrays for different frequency bands mounted about theprime axis of the EVM top cover, a VHF feed array having a dipoleelement mounted at substantially the center and near the edge of eachside of the EVM top cover; wherein one of the plurality of feed arrayscomprises four arms arranged in orthogonal turnstile relationship aboutthe prime axis, each arm having an outermost end located substantiallyat the center and near the edge of a different side of a substantiallyrectangular EVM top cover, the VHF feed having first and second coactingradiating elements mounted near said outermost ends of each of the fourarms to form four dipoles.
 9. The composite antenna feed subsystemrecited in claim 9 wherein Each of the arms of said one of the pluralityof feed arrays comprises a plurality of crossed-dipole elements.
 10. Thecomposite antenna feed subsystem recited in claim 9 wherein at least aportion of the first and second radiating elements located nearest thearms of said one of the plurality of feed arrays is surrounded by acoaxial cable.
 11. In an antenna system, a first VHF integral radiatingelement comprising: a first length extending in a perpendiculardirection relative to a ground plane of reference, a second lengthextending from the end of the first length in a direction which forms anobtuse angle with the first length, a third length extending from theend of the second length in a direction substantially perpendicular tothe first length, a dielectric support mounted between a portion of thethird length nearest its free end and the ground plane, the first,second and third lengths and the dielectric support defining a planesubstantially perpendicular to the ground plane.
 12. The antenna systemas recited in claim 11 wherein the ground plane of reference comprisesthe top cover of an EVM satellite, and further comprising: a second VHFintegral radiating element spaced from and arranged in mirror imagerelationship to the first radiating element, operative to functiontherewith as a dipole.
 13. The antenna system as recited in claim 12further comprising a feed array corresponding to another frequency bandinterposed between the spaced first and second radiating elements, andcoaxial cables surrounding portions of the first and second radiatingelements to shield the latter from the interposed feed array.